2003 Audi A3 Head Gasket replacesment, oil and Turbo!

I've just had the cylinder head replaced on an Audi 2.0 TDi, 53 reg, 92000miles. There was initially a slow coolant leak which developed and the garages conclusion was the head gasket had gone and needed to be replaced.
They did this and charged £650. Within a week the same symptoms happened.
This time they sent the head for an x-ray and ultra-sound and it revealed a hairline fracture in the head itself. They replaced this at a cost of £1044 inc VAT
Both times the car was in dock for a week.
Having collected the car last week, within 100 miles a whining sound developed on acceleration, after a further 50 miles the car lost power, still ran but at 20mph and the upshot is the TURBO has blown now.
The garage say its just a coincidence and are now replacing the turbo at a cost of £863.
Query is if the engine oil should have been changed and flushed when the head was replaced to remove shards etc.
Could dirty oil have been caused by the Head work and then caused the turbo bearings to blow?